Make Sber1/2 pref metrics into Nullable Booleans so we can track how often these prefs are unset.

components/safe_browsing_db/safe_browsing_prefs.h

Index: components/safe_browsing_db/safe_browsing_prefs.h
diff --git a/components/safe_browsing_db/safe_browsing_prefs.h b/components/safe_browsing_db/safe_browsing_prefs.h
index da25461697ea0949598bce3a39b12f1fa675afdf..54666f3b80b7a2b7b29142f1c49fac0f6b4df1f4 100644
--- a/components/safe_browsing_db/safe_browsing_prefs.h
+++ b/components/safe_browsing_db/safe_browsing_prefs.h
@@ -23,6 +23,14 @@ extern const char kSafeBrowsingScoutReportingEnabled[];
 // affects which of SafeBrowsingExtendedReporting or SafeBrowsingScoutReporting
 // is used.
 extern const char kSafeBrowsingScoutGroupSelected[];
+
+// Boolean indicating whether the user has ever seen a security interstitial
+// containing the legacy Extended Reporting opt-in.
+extern const char kSafeBrowsingSawInterstitialExtendedReporting[];
+
+// Boolean indicating whether the user has ever seen a security interstitial
+// containing the new Scout opt-in.
+extern const char kSafeBrowsingSawInterstitialScoutReporting[];
 }
 
 namespace safe_browsing {